1. Field of the Invention
Presses particularly suitable for the extraction of fruit juices and comprising a vessel which has perforated walls, and into which the material to be pressed is introduced for compression by means of a worm, have been known for a very long time. There are numerous variants of this general principle, particularly in respect of the form of the central shaft worm and the shape of the compression vessel itself. Whatever these variants, all presses of this type have the major disadvantage that the material to be pressed is triturated between the periphery of the central worm and the perforated walls of the vessel, because friction unavoidably occurs against the turns (i.e. vanes) of the worm and against the perforated walls, which act like a grater.
These disadvantages are very serious, as they entail the fragmentation of solid material and the crushing of pips or herbaceous pieces, thus giving rise to the release of oil producing a bitter taste and giving the extracted juice an astringency incompatible with good quality.
2. Description of the Related Art
French patent application FR-A-74/09591 (published under No. 2,263,884) describes a juicing press having a compression vessel (i.e. a so-called "pressing vessel") whose walls are perforated and which has a cylindrical shape, and also having a central helicoidal screw which itself is cylindrical but whose central shaft has a plurality of parts of different conicities.
French patent application FR-A-82/03408 (published under No. 2,522,585) describes a press having a first cylindrical part, a second frustoconical part, and an axial helicoidal screw whose turns are themselves cylindrical on a likewise cylindrical hub.
French patent application FR-A-83/05068 (published under No. 2,543,487) describes a press having a compression vessel comprising a first cylindrical part, a second frustoconical part connected to the first part, and a third, likewise cylindrical part connected to the frustoconical part, while the helicoidal screw again has a cylindrical contour.
It will be seen that in all cases the pressure exerted on the material is generated by the rotating helicoidal screw, which is the source of the disadvantages described above, the seriousness of which is such that the use of presses of this type for producing grape juice intended for making cognac has been banned.
The idea was then conceived of eliminating trituration by making harmless the helicoidal conveyor screw turning in the axis of the compression vessel. For this purpose the helicoidal screw is mounted for axial movement as well as for rotation in the direction applying a thrust (i.e. imparting pressing) to the material. When this screw encounters a predetermined resistance, it retracts to a starting position and then, on cessation of the rotation, the screw is pushed, without turning, against the material already pressed, while fresh material to be pressed is introduced and the screw is retracted while turning it in the opposite direction to the thrust, so that it is "unscrewed" in the fresh material, whereupon the cycle is repeated.
This arrangement provides an improvement over previous presses, but still has a considerable disadvantage, because the pressure is exerted in the same direction as the introduction of the material, that is to say in a so-called "upstream-downstream" direction, while in addition this pressure is predominant in the center of the vessel, the necessary back pressure being obtained by means of a gate disposed across the end of the vessel opposite to that where the material to be pressed enters.
For the record, mention may be made of the press which has a compression vessel containing two helicoidal screws having inverse pitches, on each of which is fixed a plate having the same section as the vessel and serving as a nut when the vessel is turned with the screws held fixed, because the plates move towards one another to press the material placed between them, or move apart to free the material, depending on the direction in which the compression vessel is driven.
A press of this kind, which has been known for many years, also has a very poor output because the time required for a pressing operation is three and one-half hours and this period of time gives rise to the oxidation of the tannin and of all oxidizable substances, including those imparting aroma, the whole operation resulting in a flat, tasteless and odorless juice when the material pressed consists of wine grapes.
In this connection, it has consequently been found that pressing a heterogenous mixture is a difficult operation if it is desired to obtain a good yield, that is to say the extraction of at least eighty per cent of liquid fraction with twenty per cent of solid fraction, while obtaining a good quality at an economic price.